The Khmer Rouge's executioner-in-chief, the prison boss allegedly responsible for the torture and murder of more than 12,000 people, appeared in court today to express his "excruciating remorse", asking that he be allowed to meet his victims' families to apologise in person.

Kaing Guek Eav, known as Comrade Duch, told a courtroom packed with about 600 people – many of them survivors of the Khmer Rouge regime – that he took full responsibility for the torture and murders that occurred at the Tuol Sleng prison in the 1970s.

"I am solely and individually responsible for the loss of at least 12,380 lives," he said. "These people, before their deaths, endured great and prolonged suffering and countless indignities. I … forever wish most respectful and humble apologies to the dead souls.

"As for the families, I [am] asking you to kindly leave your door open for me to make my apologies. May I meet with you to allow me to share your intense and enduring sorrow any time in order to express my most excruciating remorse?"

The apology, broadcast live on national TV, left many Cambodians cold. Bou Meng, one of only a dozen people to have walked out of Tuol Sleng alive, said he doubted Duch's sincerity.

"We've wept together," he said, "I know my tears are coming from sorrow. But I don't know about Duch's tears."

As a child, Norng Charnpal was rescued, filthy, starving and frightened, from Tuol Sleng when it was liberated. His mother died there. He told the Guardian he did not want Duch to apologise.

"I don't want to hear this. It is not real and it is not enough for my family. Look at him, he is an old man, he has had a long life. The way he talked, I do not believe he is genuine."

Dressed in a carefully ironed blue shirt, Duch, a former mathematics teacher, spoke calmly and coldly, his evidence littered with casual references to "the wishes of the party".

As head of Tuol Sleng prison, a converted high school also known as S-21, Duch explained that his role was to "smash" people presumed disloyal to the Khmer Rouge movement. Every prisoner was assumed guilty, Duch explained, effectively "already dead".

They were to be tortured for false confessions, usually that they were traitors working for the CIA or KGB, through electric shocks, beatings and whippings, water-boarding, having fingers cut off or toenails pulled out. The victims were then executed, most driven to nearby Choeung Ek, the Killing Fields, where they were bludgeoned to death with ox-cart axles and buried in mass graves.

"Those people were the innocent, the clean, the very honest," Duch admitted. "I don't believe they had committed any wrongdoing, as they were accused."

Speaking from a handwritten speech that ran to more than 10 pages, Duch said he found himself unwittingly caught up in a revolution he came to despise, and was forced to do his job at Tuol Sleng against his wishes, out of fear he would be killed if he refused.

"I could not withdraw from it … I am very terrified."

Earlier in the day, prosecutors asked that Duch be jailed for 40 years – in effect, a life sentence for the 67-year-old. He will be sentenced next year.

The lead prosecutor, William Smith, refuted Duch's claim that he was acting only out of fear for his own life, telling the court "the accused was neither a prisoner, nor a hostage, nor a victim. He was an idealist, a revolutionary, a crusader … prepared to torture and kill willingly for the good of the revolution."

Throughout the trial, Duch has listened attentively but impassively as the evidence of the murderous regime he oversaw is laid bare before the court. As the court heard this week of his orders that inmates who soiled themselves be forced to eat their excrement, Duch appeared inscrutable, taking meticulous notes of all that was said.

Thun Saray, head of the Cambodian Human Rights and Development Association, said no Khmer Rouge victims or victims groups had expressed any desire to meet Duch to accept his apology.

He said a truth and reconciliation commission-style body, where victims could face accusers and seek apologies and explanations, had been proposed for Cambodia, but rejected by the people.

"The majority of people are not interested in forgiveness. They want to see justice," Saray said.

The ultra-communist Khmer Rouge ruled Cambodia for four years between 1975 and 1979. The regime killed – through starvation, overwork, disease and murder – an estimated 1.7 million people, one-quarter of the country's population.

Duch is the first Khmer Rouge cadre to face trial. Four more senior leaders – including the regime's former second-in-command, Nuon Chea – are in jail awaiting trial, but there are concerns they may not live long enough to face a courtroom in 2011. A report this week by the Open Society Justice Initiative has said allegations of corruption among court officials, and the Cambodian government's open resistance to more trials, could derail the trial process.